Writing in the journal Proceedings of the Royal Society B, English researchers say they have found evidence that males can adjust the speed and effectiveness of their sperm based on whether they find their mate attractive.

The study, conducted on chickens known as red junglefowl, adds to the growing body of evidence that males from promiscuous species - including humans - increase the chances of fertilization when the female is deemed to be attractive.

Dr Charlie Cornwallis, from Oxford University, said that in the case of the junglefowl, attractiveness is determined by the colorfulness and vibrancy of the female chicken's comb, which is genetically correlated to the number and viability of the eggs she lays.

For the study, the researchers collected ejaculate from dominate and subordinate red junglefowl males. The males had either just mated with attractive or unattractive females. They then separated the sperm from the seminal fluid and analysed the quantity and characteristics of both.

"There was a strong relationship between sperm velocity and the volume of the ejaculate sperm came from [and the] males allocated larger ejaculates to attractive females," noted Cornwallis. Although the exact mechanism remains a mystery, the researchers have an intriguing theory.

"Males may alter the velocity of sperm they allocate to copulations by strategically firing their left and right ejaculatory ducts, which can operate independently," hypothesizes co-researcher Dr Emily O'Connor. Thus, stimulation from sexy, attractive females leads to the double firing.

"Furthermore," the researchers add; "differential firing of left and right ejaculatory ducts may contribute to how males strategically change the number of sperm in their ejaculates, a phenomenon that is widespread, but for which the mechanism remains unknown."

The researchers conclude that much of this process, including the detection of attractive females, can occur on a subliminal basis.